For many years now, felling heads have replaced the traditional chainsaw in most logging operations where trees are of medium diameter. The felling head is usually used to restrain, cut, accumulate and pile trees in a single, quick operation. The felling head is normally mounted on the end of a hydraulic crane or manipulator, which is, in turn, mounted to a tractor or tracked vehicle. The felling head is a much more efficient device than the traditional hand-carried chainsaw and, surprisingly, it causes less damage to the environment because felled trees are not dragged on the forest floor, crushing and uprooting young stems, saplings and undergrowth.
Some felling heads are especially well-suited for manipulating trees mainly because they can be tilted back and forth as well as sideways. In this specification, the expression "tilted back and forth" refers to the pitch of the frame as illustrated in FIG. 1. Similarly, the expressions "tilted sideways" or "side tilt" should be construed to refer to the roll of the frame about an axis of rotation transverse to the frame as illustrated in FIG. 6.
The back and forth tilting motion is achieved by a hydraulic actuator normally installed on the manipulator arm (or crane). The double attachment of the felling head to the manipulator arm allows the felling head to pivot about one of the attachment points while the hydraulic actuator pushes or pulls on the other attachment point. This back and forth tilt (or pitch) makes it possible for the felling head to position itself parallel to the tree and, after the cut, to lay the tree on the ground.
In addition to the back and forth pitch, the sideways tilting motion (i.e. the roll) gives the felling head greater maneuverability in gripping, cutting and manipulating trees and thus increases the potential for producing larger piles of trees in a given period of time without having to constantly readjust the position of the tractor. Thus, a highly maneuverable felling head increases the efficiency of the logging operation. Furthermore, since the felled trees can be easily manipulated and piled in a convenient location for the transporter to pick them up, the harvesting minimizes the damage to neighboring trees and saplings.
Although the present description deals primarily with the gripping, cutting and manipulating of trees, it should be appreciated by a man skilled in the art that the felling head of the present invention could also be used to grip, cut and manipulate wooden poles (e.g. telephone poles or power-line poles) or, with a different type of saw blade practically any tree-like or pole-like object.
In the prior art, many different types of mechanisms have been used to create the side tilting motion of the felling head. Some prior art systems use a pair of hydraulic actuators mounted on the felling head that are linked to a large gear, itself linked to a central gear around which the tilting motion is achieved. The hydraulic actuators push and pull simultaneously on the large gear, thereby rotating the felling head around the central gear. In this case, the rotation is limited by the travel of the hydraulic actuators to a side tilt rotation of about 180 degrees.
Other systems use a hydraulic motor and chain arrangement. A hydraulic motor with a drive shaft and sprocket is mounted on the felling head and linked with a chain to a large central sprocket located at the rotational axis of the felling head. When the motor sprocket turns, it induces a side tilting motion to the felling head. This arrangement has a much larger rotational range than the hydraulic actuator arrangement described in the previous paragraph. Its range can reach 360 degrees.
Other combinations of actuators, gear assembly, sprockets and chains are commonly found in the prior art but they all share the same feature in that the components necessary for the side-tilt rotation are housed directly on the felling head itself and in a position that is not coaxial with the side-tilting axis of rotation of the frame of the felling head. This contributes extra weight and inertia to the felling head which, in turn, reduces the maneuverability of the felling head. Not only are such cumbersome felling heads slower to rotate but the energy expenditure during operation is higher. Moreover, since the motor is located on the frame, the hydraulic lines have to be routed from the holder to the frame. When operating amid thick branches, these hydraulic lines may become snagged.
These prior art arrangements tend to be bulky, heavy and generally involve moving parts that are often exposed to the rigorous environment of the logging field. Exposed components such as gears, chains, sprockets or hydraulic actuators, are susceptible to being hit and damaged. Open moving parts are prone to becoming tangled in branches. Furthermore, dust and dirt tend to clog lubricated moving components and thus these arrangements require regular maintenance to avoid breakdowns.
Thus, there is a need in the logging industry for an improved felling head.